Conversion of salts of water soluble organic acids to pure acids has long been a problem. Simple neutralization with mineral acids or the like results in an aqueous mixture of the salt of the mineral acid and the desired organic acid. Isolation of the acids from this mixture generally results in contamination of the mineral acid salts with organics and thus to environmental disposal problems. In addition, the organic acids isolated in many cases are contaminated with salts of the mineral acid.
In order to overcome the aforementioned problems, Bodamer (U.S. Pat. No. 2,921,005) described a three-compartment electrochemical cell for neutralization of water soluble salts of weak acids to the corresponding acids. The cell used had an anode compartment, a cathode compartment, an intermediate compartment formed by two cation exchange membranes. The membranes were both sulfonated copolymers of styrene and divinylbenzene. The process was carried out by electrolyzing water on the anode to form oxygen and hydrogen ions in a dilute sulfuric acid electrolyte. Water was electrolyzed on the cathode forming hydrogen and hydroxide ions. The center compartment contained the sodium salt of a weak acid such as acetic acid. The complete process involved transferring protons from the anolyte compartment through the first cation exchange membrane into the center compartment while transferring sodium ions from the center compartment through the second cation exchange membrane to the catholyte compartment where combination with the hydroxide formed by the cathode reaction formed sodium hydroxide. It is noted in the example of the patent that the minimum voltage at 0.42 amp per square inch (asi) was 11-12 volts and this same voltage was obtained at only 0.04 asi towards the end of conversion. In addition, a small amount of the acid in the center compartment had leaked to the anode compartment. The cathode compartment contained only 0.1N (0.4%) sodium hydroxide.
A later patent to Wallman (U.S. Pat. No. 3,411,998) teaches using a cell such as described by Bodamer for reclaiming spent alkali metal carboxylate solutions. In this patent, the total reaction for the center compartment when a tribasic acid such as citric acid (Cit) is used is: EQU Na.sub.3 Cit+2H.sup.+ .fwdarw.NaH.sub.2 Cit+2Na.sup.+
The total process involves feeding the caustic formed at the cathode to an evaporator for concentration to a useful level. The membranes used in the process were typical hydrocarbon types available commercially at that time.
A later patent to Giuffrida (U.S. Pat. No. 4,057,483) attempts to improve the process of the above-mentioned three-compartment cell by adding another compartment. In this patent, a four-compartment cell is formed by interposing three cation exchange membranes between the anode and cathode. The membranes used are said to be selected from those commercially available. In this cell, the sodium salt of the weak acid is circulated through both center compartments with predominant conversion of the salt to the acid being achieved in the compartment nearest the anode. The purpose of the extra compartment is to intercept protons transported from first compartment and thus prevent neutralization of the caustic formed at the cathode with the attendant loss in current efficiency. A further purpose of the four compartment cell is to provide a buffer zone formed by the intermediate salt feed compartment. The prior art three compartment cell at low conversions not only tend to lose weak acid ions to the anolyte but at high conversions allows great loss of weak acid to the catholyte. Examples 1 and 2 of the patent show a direct comparison of the four compartment and the three compartment with 8% caustic in the catholyte. The four compartment gives 83% current efficiency while the three compartments only an average of 65%. No cell voltage information is given in any examples. Thus, overall power efficiencies can not be calculated.